yep, it's ig plus to do so on all your birds. I know with that many coming in at once, it can be hard to do, but worth every penny.
30 days is the basic rule of thumb. And if one seems sickly or shows symptoms, the clock starts over after it's all better for another 30 days, just to be safe.
Like TenneesseTruly said, I too have seen many folks get wiped out with various problems from not doing so.
Anyway, on to the hen setting thing.
They are the best incubator you can have. Keep in mind though, many especially new hens, often will just lay eggs randomly over the pen, or just not go broody for you, so # 1 , watch that.
also, trying to catch day old ducklings in a huge pen, LOL Video tape that for me, that'll be cute !
Always best, like you were saying to try to watch the hen closely, and ruffly estimate when she's due to hatch. when I did let hens set, I'd always try to pull them the last week -5 days and incubate the rest of the way.
I never let any be pen raised, though a lot of folks do.
But if you do, you need very small wire around the bottoms, watch snakes (they love them) watch drakes (they can at times kill babies on some breeds, others are just as protective as hens) To me though, yes, they are much safer in the brooder, plus that way you have a shot at the hen producing again, if she raises them though, 9 times out of 10, she's done for the year.
Get us some hatched, I sure hope to start recollecting my birds this coming season.